This invention relates generally to the field of wireless digital communication systems and, more particularly, to wireless digital communication systems that implement adaptive modulation and coding scheme and power control scheme.
Wireless communication systems, such as cellular, use a wireless link comprised of a modulated radio frequency (RF) signal to transmit data between senders and receivers. Since RF bandwidth is a scarce resource, various signal processing techniques have been developed for increasing efficiency of the usage of the available RF bandwidth. An example of such signal processing techniques is the IS-95 promulgated by the telecommunication industry association (TIA). The IS-95 standard, used primarily within cellular telecommunications systems, incorporates code division multiple access (CDMA) to carry out multiple communications simultaneously over the same bandwidth. Under the IS-95 standard, data may be transmitted over a RF link at a maximum data rate of 9.6 or 14.4 kbps for voice codec, or up to 64 kbps for packet data communication, depending on which rate set from a set of data rates is selected. Such data rates as specified by IS-95 may be suited for wireless cellular telephone systems if the typical communication involves the transmission of digitized voice or lower rate digital data such a facsimile.
The International Telecommunication Union (ITU) of the Internet Society, the recognized authority for worldwide data network standards, has recently published its International Mobile Telecommunications-2000 (IMT-2000) standard. The standard proposes so-called third generation (3G) and beyond (i.e., 3.5G, 4G etc.) data networks that include extensive mobile access by wireless, mobile nodes including cellular phones, personal digital assistants (PDAs), handheld computers, and the like. (See http://www.itu.int). The IMT-2000 standard adopts wideband direct sequence code division multiple access (W-CDMA) as a wireless access method for the proposed third generation and beyond networks and requires a maximum data rate of 144 kbps (vehicular), 384 kbps (pedestrian) or 2 Mbps (quasi-stational), depending on the environment in which wireless communication is carried out. Thus, in communication networks according to the IMT-2000 standard, communication services that require high data transmission rates, such as the multimedia communication service, are indeed feasible over RF links.
The recent phenomenal growth of Information Technology and the Internet creates a need for a high performance wireless Internet technology and has in fact promoted development of various data transmission technologies for wireless data services. One such technology is the adaptive data rate scheme in which a data rate is adaptively changed according to the receiver's RF channel condition. One of the key requirements for wireless Internet is to maximize the data throughput in a given cell or sector. The adaptive data rate scheme optimizes data throughput on average by serving multiple data receivers at maximum data rates that the receivers can accept, given their RF channel conditions. Thus, under the adaptive data rate scheme, receivers with good channel conditions receive data at higher data rates, and receivers with poor channel conditions receive data at lower data rates.
The adaptive data rate scheme is a unique technology in many aspects. Recognizing the characteristics peculiar to data services, such as traffic asymmetry and high tolerance to latency, the adaptive data rate scheme decouples data service from voice service. Two-way conversational speech requires strict adherence to symmetry on the downlink (forward link) and uplink (reverse link) traffic and is very delay sensitive. For instance, latencies above 100 ms are intolerable and make speech unintelligible. It is also true that a relatively modest data rate is sufficient for high quality voice service. On the other hand, data services are, depending upon an application implemented, generally characterized by heavy downlink traffic and light uplink traffic and have high tolerance to latency. For high-speed data downlinked at 1 Mbps, for example, 100 ms represents just 100 kb or 12.5 kbytes, and even latencies of a couple of seconds are hardly noticeable. The decoupling of voice and data services reduces design complexities of Physical Layer because the Layer is relieved from difficult system load-balancing tasks, such as one for determining whether voice or data calls have higher priority.
The adaptive data rate scheme is usually implemented with time division multiple access (TDMA) scheme in order to serve multiple receivers simultaneously at different data rates. TDMA scheme subdivides the available frequency band into one or several RF channels called “frames.” The frames are further divided into a number of physical channels called “time slots.” The adaptive data rate scheme takes advantage of the characteristic of the TDMA channel that data rate control is possible on each slot. The adaptive data rate scheme may be implemented with code division multiple access (CDMA) schemes such as the time-slotted CDMA scheme.
Implementation of the adaptive data rate scheme requires measurement of a RF channel condition and determination of a maximum data rate feasible under the RF channel condition. For this and other useful purposes, at least one pilot burst is inserted into each time slot. Upon reception of the first pilot burst in each time slot, a receiver estimates its downlink channel condition and computes a maximum data rate that the estimated channel condition can support at a given level of error performance. The receiver then reports the calculated data rate to the sender. In order to transmit data to the receiver at the reported data rate, the sender selects a modulation scheme and a coding rate that can achieve data transmission at the reported data rate.
When there are multiple receivers requesting data simultaneously, the sender needs to have a scheduling functionality (a scheduler) that determines the order in which the receivers are served. Various scheduling algorithms have been proposed and used. Basically, these conventional algorithms try to achieve the same goal, i.e., optimizing the overall data throughput. To achieve the goal, these algorithms are designed to serve receivers with good channel conditions favorably to those with poor channel conditions. Thus, under the typical conventional scheduling algorithm, receivers with good channel condition are served first, and receivers with poor channel condition are served later. FIG. 1 shows a simplified graphical representation showing implementation of the adaptive data rate scheme. In FIG. 1, an access point (AP) has three sets of data ready to be transmitted to three access terminals (AT) 1, 2 and 3, respectively. The ATs (1-3) have already estimated their downlink channel conditions based on the received pilot bursts sent from the AP and returned to the AP data rates that they can accept. Suppose that the AT (1) has the best channel condition among them, the AT (2) has the next best condition and the AT (3) is the last. Accordingly, the AT (1) is requesting the highest data rate among them, the AT (2) is requesting a lower data rate and the AT (3) is requesting the lowest data rate. According to the above conventional scheduling algorithm, the AT (1) is served first, the AT (2) is next, and the AT (3) is last as shown in FIG. 1.
Another feature of the adaptive data rate scheme is that ATs transmit data at the maximum power level. The higher the transmission power level is, the better the channel condition will be. For instance, a signal to interference ratio (SIR) is one of the parameters indicative of channel condition. If a signal is transmitted at a higher power level, the SIR will improve or become higher because signal “S” becomes larger in the ratio. If the SIR becomes larger, the signal may be transmitted at a higher data rate. Thus, transmitting data at the maximum power level serves the very purpose of the adaptive data rate scheme. On the other hand, however, a signal to one AT is at the same time interference to other ATs being receiving signals from other APs. Thus, one AP transmitting data to one of its ATs at the maximum power level will improve the SIR of the AT but deteriorate the SIRs of other ATs receiving data from other APs.
FIG. 2 is a simplified graphical representation showing geometrical relationship between two access points and four access terminals. An AP 1a forms a virtual communication zone (“zone”) A. An AP 1b forms its zone B. Each of the zones A and B represents an area within which it is feasible at a given level of error performance for ATs to communicate with the AP in the zone. The size of zone changes according to a change in transmission parameters, such as transmission power. If transmission power becomes high, the zone becomes larger. If transmission power becomes low, the zone becomes small. Although not shown, around the APs 1a and 1b, other APs exist and form their own zones. Also, four ATs 10a, 10b, 10c and 10d are located within the zones A and B. The AP 1a is serving the ATs 10a and 10c. The AP 1b is serving the ATs 10b and 10d. The ATs 10a and 10b are located near the fringes of the zones A and B and nearly equidistant from the APs 1a and 1b. The signal transmission from the AP 1a to the AT 10a manifests itself as interference to the signal reception by the AT 10b. Likewise, the signal transmission from the AP 1b to the AT 10b manifests itself as interference to the signal reception by the AT 10a. For the ATs 10a and 10b, because of their relative positions with the APs 10a and 10b, the signal level and the interference level are nearly equal, and thus the SIRs of the ATs 10a and 10b are low. As discussed above, under the typical conventional scheduling algorithm for the adaptive data rate scheme, ATs with good channel conditions are served first, and those with poor channel conditions are served later. Accordingly, the ATs 10a and 10b receive data less frequently or may never be able to receive data at worst until they change their relative positions with the APs 1a and 1b. 